Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The rotor rotates within (or around) the stator when the motor is energized with a driving waveform. Induction motors, sometimes referred to as asynchronous motors, are a type of electric motor wherein power is supplied to the rotor by means of electromagnetic induction rather than by means of direct electrical connections to the rotor.
As with synchronous motors, the driving waveform supplied to an induction motor's stator creates a magnetic field that rotates in time with the AC oscillations of the driving waveform. The induction motor's rotor rotates at a slower speed than the stator field. This difference in rotational speed, also referred to as “slip,” “slip frequency,” or “slip speed,” results in a changing magnetic flux in the rotor windings that induces currents in the rotor windings. The induced current in turn generates magnetic fields in the rotor windings that oppose the rotating magnetic field created by the stator, thereby inducing rotational movement in the rotor. The rotor accelerates until the magnitude of induced rotor current and rotor torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates at less than synchronous speed during normal forward operation.
When the frequency of the driving waveform falls below the rotor frequency, such as where the programmed speed of the motor is reduced by a motor controller during operation, the rotating magnetic field created by the stator induces rotational pressure on the rotor that opposes the rotor's movement and reduces motor speed. In this braking mode of operation, the inertia of the rotor and applied load induces voltage in the stator that may energize external motor components, such as a DC bus supplying power to the motor.
A known method of controlling electric motors is field oriented control or “FOC.” Field oriented control involves controlling a motor using three motor input variables including voltage magnitude, voltage angle and frequency. Because stator current is closely related to output torque and other operating characteristics of the motor, it is desirable to use field oriented control methods to operate the motor according to a target stator current. At high motor speeds, however, manipulating the stator current using field oriented control becomes less effective because the motor operates at or near a maximum voltage level, eliminating or reducing one of the input variables (voltage magnitude).
The above section provides background information related to the present disclosure which is not necessarily prior art.